The present invention generally relates to a capacitive load driving apparatus and, more particularly, relates to an apparatus for driving capacitive loads operated at a high voltage and arranged in a matrix.
A circuit for driving a capacitive load is generally constituted by a source side switch for charging the load and a sink side switch for discharging the charged load. As the prior art relating to such a capacitive load driving circuit, known is a technique disclosed, for example, in Japanese Patent Unexamined Nos. 60-208119, 58-221517, etc. To drive a capacitive load, generally, it is preferable to apply a rapid rise pulse as described. For example, in the above Japanese Patent Unexamined No. 58-221517, and to this end, a driving circuit having a large current driving capability is required. In the driving circuit disclosed for this purpose in the above Japanese Patent Unexamined Nos. 58-221517 and 60-208119, a thyristor is used as the source side switch S.sub.p. As the sink side switch, on the other hand, a bipolar transistor is widely used.
Examples of a capacitive matrix load include an EL panel, and so on. An EL panel is driven with AC power because it initiates light emission at a high voltage of about 200 V, and it has a polarization effect. This point is described in detail, for example, in Japanese Patent Unexamined Nos. 52-123883 and in an article entitled "Bidirectional Push-Pull Symmetric Driving System of TF-EL Display", SHARP TECHNICAL JOURNAL, No. 38, pp. 61-65, 1987. In general, an EL panel is provided with scanning side electrodes to which a high voltage is successively and selectively applied and data side electrodes to which a relatively low voltage is applied in accordance with light-emission and non-light-emission data and in synchronism with the application of the high voltage to the scanning side electrodes, those scanning side electrodes and data side electrodes being arranged to intersect each other and to form an EL layer therebetween. Each of the portions enclosed by those scanning side electrodes and data side electrodes forms one unit picture element which is a capacitive load equivalently.
A problem has been newly found which is caused, as follows, when the scanning side electrodes in a capacitive matrix load is driven by use of such a driving circuit as disclosed in the above Japanese Patent Unexamined Nos. 58-221517 and 60-208119.
That is, with the tendency to make an EL panel large-scaled, it has become preferable to enlarge the current driving capability of a driving circuit. In this regard, in the conventional driving circuit, there is a possibility that a transistor acting as a sink side switch deviates from an area of safe operation (hereinafter abbreviated to "ASO") when a picture element is discharged after it has been charged with a positive high voltage V.sub.HP, because a discharge current of the picture element is begun to flow in the condition that the positive high voltage V.sub.HP is being applied to the collector of the transistor. To prevent this phenomenon, it is necessary to restrict the discharge current by any means. When a picture element selected by the sink side switch is charged with a negative high voltage V.sub.HN, on the other hand, the absolute value of the negative high voltage V.sub.HN, that is .vertline.V.sub.HN .vertline., is smaller than the value of the positive high voltage V.sub.HP, and when the picture element emits light, the voltage is borne by the picture element side. Accordingly, the problem with respect to the ASO is reduced, and a large current driving capability is thereby required in order to obtain luminous operation. That is, the requirement to the sink side switch is reverse to that in the case where the picture element is charged with the positive high voltage V.sub.HP . In order to satisfy those opposing requirements, it is necessary to make the transistor acting as the sink side switch large-scaled. To make the transistor large-scaled is not preferable because the area of a chip becomes large in size when the driving circuit is made in the form of an integrated circuit.